Current residential and commercial fence and rail materials are made of traditional lumber, metal or polymers including thermoplastics. Typical thermoplastics used in these applications are PVC (polyvinyl chloride) and polyolefins such as polyethylene and polypropylene. Thermoplastics typically do not have the strength and rigidity of wood and lumber and, therefore, the rail for the fence and railing needs a steel or aluminum reinforcement channel inside the rail. Metal fence and rail materials, as well as the metal reinforcements used with current PVC fence and rail materials, are prone to corrosion attack, and lose strength in long-term endurance tests. Additionally, where dark-colored thermoplastics materials are employed, thermal expansion problems can arise due to differences in expansion between opposing sides of the product when the product is exposed to sunlight. Since the dark color absorbs heat more readily on the sun-facing side of the product, the resultant uneven heat buildup causes the rail to deform. An additional problem is the lack of long-term stiffness of polymeric products, which has limited the rail span between the posts to lengths less than traditional lumber and metallic rails.
One solution to the stiffness problem is to add filaments to the thermoplastic resin during manufacturing. These filaments typically are added as small-length (e.g., less than ½-inch long) chopped filaments made of any of a variety of materials. The resulting thermoplastic profiles can have increased strength and stiffness as compared to un-reinforced profile products. Typically, the short-filament reinforcement is achieved using extrusion techniques. Alternatively, continuous or long filament reinforcement of thermosetting and thermoplastic profiles has been achieved using pultrusion processes.
Conventional continuous filament reinforced thermoplastic profile products produced using conventional pultrusion processes still suffer from deficiencies in mechanical properties in the cross direction (e.g., flexural strength, tensile strength, impact strength, and compression strength), due typically to poor bonding between the substrate resin and the filaments. This poor bonding is primarily due to poor resin wet-out of the filaments by the resin. Thus, for applications using mechanical fasteners, the profile products have low screw and nail holding power. Additionally, cracking, splitting and separation of filaments in these profiles can easily occur during transportation, application, and installation.
As a result, in order to achieve desired high mechanical strength levels, filament reinforced thermoplastics profiles produced using conventional pultrusion processes, are designed to have thicker walls, and/or higher glass filament loading. Both of these approaches may result in a higher than desirable profile weight.
Additionally, conventional pultrusion processes for incorporating long filaments are relatively slow, and thus result in an undesirably low output rate (e.g., frequently on the order of only 2-3 feet per minute). The slowness of the process is due to the time required for (1) melting of the thermoplastics, and (2) wet-out of the melted thermoplastics around the filaments. Filament wet-out with thermoplastic resin is typically poor, even at higher temperature and longer residence times, due to the relatively high viscosity of thermoplastic materials. Further, incompatibility between the thermoplastic resin and the filament material can also lead to poor wet-out.
Thus, to achieve a desired total production rate with current pultrusion processes, additional machines must be employed. The additional machines, however, take up additional manufacturing floor space and involve larger amounts of capital investment, thus leading to increased costs.
Alternatively, conventional filament reinforced thermoplastic profile products produced using conventional extrusion processes suffer from deficiencies in mechanical properties in the cross direction because current processes only allow for the incorporation of very short filament lengths. This is because conventional extrusion of long filament reinforced thermoplastic profile products typically involves the use of an intermeshing twin screw extruder, whose intermeshing screws act like scissors which chop the filaments to short lengths, regardless of the length of the filaments added to the resin. Such processes can result in filament lengths of about one tenth of their originally added length. Where discrete-length filaments (in lieu of continuous filaments) are introduced into the extrusion flow, original filament lengths are limited to about ½-inch or shorter in length. If the filament length is too long (i.e., over ½-inch), the filaments will form a bridge at the introduction hopper, clogging the hopper and inhibiting feeding of the filaments into the extruder. Where filaments of less than ½-inch in length are used, the intermeshing twin screws can chop the filaments to even shorter length. Such short filament lengths are undesirable for use as reinforcement for thermoplastic products because they do not provide the enhanced strength that is desired. Additionally, filament loading using such processes is low.
Thus, there is a need for high strength thermoplastics products which incorporate a long or continuous filament reinforcement scheme. There is also a need for a process for producing such products in a fast and economical manner so as to make high-strength reinforced products commercially viable.